A superconducting material is dependent on three parameters to achieve or maintain its superconducting state. If any of these parameters exceeds a critical value, the superconducting state ceases and the material resumes its normal conducting state. The three parameters are temperature, current density and magnetic flux density. All these parameters have been used to achieve the desired current limitation.
An example of how to use several of the critical parameters in combination in order to produce a current limiter is described in an article in Journal of Applied Physics, 49 (4), April 1978, pages 2546-2550 by K. E. Gray and D. Fowler entitled "A superconducting fault-current limiter". This current limiter comprises two parallel-connected resistors. One resistor consists of a material which may become superconducting and which is located in a cryostat, i.e. a cryotank, in which the low temperature which is necessary to obtain a superconducting state can be attained, while at the same time the resistor is dimensioned such that the current at which the current limiter is to enter into operation gives a current density which is well below the critical density and such that also the normally surrounding magnetic flux density is lower than the critical density. In the embodiment described in the above-mentioned article, the resistance is changed from practically zero at a superconducting state to the order of magnitude of 13 k .OMEGA. when the superconducting state ceases. The second resistor is dimensioned such that its resistance is considerably lower than the resistance of the superconducting resistor when this is no longer superconducting; in the case referred to the resistance is about 3.6 .OMEGA.. Under normal circumstances of the network, the superconducting resistor is kept in a superconducting state, i.e. its resistance is zero whereby current only flows through this resistor and no voltage drop occurs across the two parallel-connected resistors. Now, if the mains current because of a short-circuit or otherwise exceeds a permissible value corresponding to the value at which it is desired that the current limiter should start operating, the superconducting resistor in the described device is changed by a magnetic field partially changing the magnetic flux density around the conductor, thus obtaining a partial increase of the resistance which also results in an increase of the temperature in the resistor. Before long this will lead to the superconducting state being terminated and a predominant part of the current having to pass through the parallel-connected resistor.
Only using the critical current density to cause a superconducting material to pass from superconducting to non-superconducting state is described in DE-OS 2 712 990 entitled "Anordnung zur Ueberstrombegrenzung in elektrischen Energieversorgungsstrecken". This patent specification describes a current limiter in the form of a "superconducting cable" consisting of conductors of a material that may become superconducting and of conductors ("Matrix-/Tragermetall") dimensioned such that the superconducting cable constitutes a current-limiting element when the parallel-connected superconducting conductor is no longer superconducting. The superconducting conductor(s) is (are) formed with certain regions having area reductions ("Einschnurungen") whereby the critical current density is exceeded when the current carried through the cable exceeds the current at which the current limiter is to start operating. One problem in connection with the superconducting cable described, which is passed over in silence, is what happens to the heat development that occurs inside the cable when changing from a superconducting to a normally conducting state.
The fact that current limiters based on the two different states of a superconductor have not been used to any significant extent is due to a number of reasons. Up to a few years ago, the critical temperature lay at very low values, which entailed expensive and difficultly-manageable cooling devices, normally using helium as coolant. In part, the low use is also due to the complexity of the equipment involved, for example the equipment described in Journal of Applied Physics.
The discovery of new materials which become superconducting at a considerably higher temperature, which, for example, enables the use of liquid nitrogen as coolant, has, of course, also a positive effect on existing technical solutions.